The present application claims the right of priority under 35 U.S.C. xc2xa7119 (a)-(d) of European Patent Application No. 02013461.5, filed Jun. 14, 2002.
The present invention relates to an improvement of a toluenediisocyanate (TDI) recovery and purification process which uses a heat integrated system comprising two distillation columns connected in series for the fractionation of a crude isocyanate stream. The heat integration enables energy efficient operation for various feed rates, compositions and product specifications. The process of the present invention benefits from the ability to achieve a lower total manufacturing cost.
The present invention relates to a process wherein toluenediamine is reacted with phosgene in the presence of a solvent solution in the liquid phase or wherein toluenediamine is reacted with phosgene directly in the gas phase with a solvent used in the quench cooling of said reaction; excess phosgene is then partially or completely removed from the resulting reaction mixture and the dephosgenated crude distillation feed is fed to a fractionation process wherein four fractions are recovered
1. a phosgene-enriched low-boiler product, which is recovered and returned to the dephosgenation or excess phosgene recovery process,
2. a relatively pure solvent product (less than 100 ppm by weight TDI) which is then reused in the phosgenation or excess phosgene recovery process,
3. a high-boiler (polymeric isocyanate, hydrolyzable chloride compounds (HCC), and other non-volatiles) enriched bottoms product which is sent to a residue removal system for the further recovery of volatiles,
4. and optionally an isocyanate-enriched product stream
The field of art to which this invention pertains is a process for the purification of toluenediisocyanate (TDI) mixtures. TDI mixtures are generally produced by reacting toluene with nitric acid to yield dinitrotoluene (DNT), hydrogenating the resultant dinitrotoluene (DNT) to yield toluenediamine (TDA) and reacting the toluenediamine (TDA) with phosgene to give toluenediisocyanate (TDI). Toluenediisocyanate (TDI) is a commercial available material particularly useful in the preparation of polyurethanes, polyurea and polyisocyanurate polymers, especially foamed polymers.
DE-A1-3736988 teaches that organic mono- or poly-isocyanates are continuously prepared by reacting the corresponding mono- or poly-amine dissolved in an inert organic solvent with phosgene also dissolved in an inert organic solvent at a temperature under 150xc2x0 C. The amine and phosgene solutions are combined and allowed to pass through one or more reaction columns connected below to above in series and having at least 10 chambers in total separated from each other by perforated plates, the holes of which preferably have a maximum diameter of 20 mm.
EP-A1-570799 teaches that production of aromatic diisocyanates is effected by reaction of diamines and phosgene. The phosgene and diamine are at above the boiling temperature of the diamine and the reaction has an average contact time of 0.5-5 seconds. The mixture is continuously passed through a cylindrical reaction space at 200-600xc2x0 C. to complete the reaction with avoidance of back mixing. The gas mixture is then cooled to condense the diisocyanates, with the temperature being maintained above the decomposition temperature of carbamic acid chlorides corresponding to the diamines used. Uncondensed diisocyanate is washed out of the gas mixture with an inert solvent, and the inert solvent is recovered by distillation.
The Polyurethane Handbook (Oertel, G. (Editor), Polyurethane Handbook, Munich, Germany: Hanser Publishers, 1985, pp 62-73) gives a description of a state of the art for the phosgenation and distillation process for the production of toluenediisocyanate. In the distillation process, the solvent is completely removed from the crude TDI mixture as the top product from a solvent column, with this solvent being returned to the phosgenation or to the excess phosgene recovery. The remaining crude isocyanate bottoms stream from the solvent column is sent to a pre-flasher where two products are achieved: a isocyanate rich overhead product and a residue-enriched bottoms stream which is fed to the residue removal. In the residue removal, the volatiles are then removed from this residue-enriched stream and condensed. The condensed volatiles from residue removal together with the condensed overhead stream from the pre-vaporization are then combined and fed to an isocyanate column. In the isocyanate column, the product isocyanate is recovered as a top stream while a high-boiler enriched bottoms stream is returned to the pre-vaporization step. This process is limited by the fact that the complete solvent removal is performed in one solvent column. While it is known that TDI yields are negatively affected by higher temperatures, complete solvent removal necessitates operating under relatively low pressures to achieve sump temperatures low enough to prevent a loss of yield, thus necessitating a large column. Moreover, the long residence time of isocyanate together with residue in heating zones can lead to a higher rate of residue formation. Finally, condensation of the overhead stream from the pre-vaporization before feeding to the isocyanate column is energy inefficient.
In Industrielle Aromatenchemie (Franck H.-G. and Stadelhofer J., Industrielle Aromatenchemie. Berlin, Germany: Springer Verlag, 1987, p 253) a second state-of-the-art process is described. In the described process, the crude TDI-solvent mixture is fed to a two-step pre-vaporization step resulting in a low-boiling overhead vapor product and solvent-free residue-enriched bottoms product which is fed to the residue removal. In the residue removal process, the volatiles are then removed from this residue-enriched stream and condensed. The overhead product from the pre-vaporization is fed to a solvent column. In the solvent column the solvent is completely removed as the top product, with the solvent being returned to the phosgenation or to the excess phosgene recovery. The remaining crude isocyanate bottoms stream from the solvent column is fed along with the condensed volatiles from residue removal to an isocyanate column. In the isocyanate column, the product isocyanate is recovered as a top stream while a high-boiler (polymeric isocyanate and hydrolyzable chloride compounds (HCC), and other non-volatiles) enriched bottoms stream is returned to the pre-vaporization step. This process is also limited by the fact that the complete solvent removal must be performed in one solvent column. As in the process described in the Polyurethane Handbook, complete solvent removal necessitates operating under relatively low pressures to achieve sump temperatures low enough to prevent a loss of yield, resulting in a large solvent column. However, this process, in comparison with the former process achieves a reduced residence time of isocyanate together with residue in heating zones possibly leading to a lower rate of residue formation. Moreover, because there is no needless condensation of a vapor feed to the isocyanate column, this process will be more energy efficient.
From Chem. System""s PERP Report for TDI/MDI (Chem. Systems, Process Evaluation Research Planning TDI/MDI 98/99S8. Tarrytown, N.Y., USA: Chem. Systems, 1999, pp 27-32) for TDI/MDI it can be learned, that the fractionation of a crude TDI distillation feed product can be completed in the following manner. Normally, the liquid product from the dephosgenation stage is sent to a pre-vaporizer which produces a residue-rich liquid phase as a bottom product and a vapor-phase product containing mainly solvent and isocyanate as an overhead product. The bottom product from the pre-vaporization is sent to a process for the removal of volatile compounds from the reaction residues (residue removal). The volatile components removed in the residue removal stage as well as the vapor-phase product from the pre-vaporizer are sent to a solvent column, where an initial separation of the isocyanate from solvent is completed as well as the removal of any remaining phosgene. The resulting products are a phosgene-enriched top product, a relatively pure solvent stream as an intermediate product and an isocyanate-enriched bottoms product. The phosgene stream is then returned to the dephosgenation process or to the excess phosgene recovery process. The solvent product is then used in the phosgenation section as well as in the excess phosgene recovery. The bottoms isocyanate-rich product is then sent to a second solvent removal column where the remainder of the solvent is removed. The top solvent product from this step, when relatively pure, can be used in phosgenation or excess phosgene recovery or can be returned to the primary solvent removal step. The final solvent-free bottoms isocyanate product is sent to an isocyanate column, resulting in an isocyanate top product and a residue and hydrolyzable chloride compound (HCC) enriched-bottom stream which is returned to the pre-vaporization or to the residue-removal stages. This process, like the process described in Industrielle Aromatenchemie, in comparison with the process described in the Polyurethane Handbook achieves a reduced residence time of isocyanate together with residue in heating zones possibly leading to a lower rate of residue formation. Additionally, like the process described in Industrielle Aromatenchemie, because there is no needless condensation of a vapor feed to the isocyanate column, this process will be more energy efficient than the process disclosed in the Polyurethane Handbook. It holds the additional advantage that the solvent removal is completed in two steps. By taking advantage of the solvent having a lower boiling point than the isocyanate, the majority of the solvent can be removed under higher pressure, therefore, reducing the necessary investment cost for the solvent removal. Additionally, the use of two solvent removal steps adds to the flexibility of operation. However, the presence of a third column adds more complexity to the process.
In fractionation, it is sometimes desirable to separate a multi-component feed stream into a number of streams containing various fractions of desirable components in the product streams. For the case of one feed stream and two product streams, the separation can be accomplished by distillate and bottoms product draw. Further separation can be accomplished by repeating the two-product stream process to either the distillate or the bottoms streams. However, the introduction of additional columns will require a corresponding number of reboilers and condensers. That requirement, in turn, requires additional operating costs as the condensing and the reboiling process is being repeated. Numerous references can be found in prior art documenting efforts to lower both capital and operating costs in the separation of several fractions from a multi-component feed stream.
One potential way to decrease the energy process is the integration of energy between two columns in a fractionation system. (Annakou, O and Mizsey, P, Rigorous Comparative Study of Energy-Integrated Distillation Schemes, Industrial and Engineering Chemistry Research, 1996, 35, pp 1877-1885). In such a configuration, the vapors from one column are condensed to provide the energy to reboil the bottoms product of the other column. This can either be performed in a process wherein the vapor of the upstream distillation column is used to reboil the bottom product of the downstream distillation column or conversely, where the vapors of the downstream column are used to reboil the bottoms product of the upstream column.
Generally, the development of the process for TDI recovery has resulted in reductions in capital investment, greater energy efficiency, and improved product yield. But, the energy consumption, capital investment and product yield is still insufficient.
In the present invention, the use of a system of heat integrated distillation columns wherein the heat of the vapor of the upstream distillation column is used to vaporize the feed to the downstream column or to reboil the bottom product of the downstream distillation column for the partial or total removal of solvent allows for a surprising reduction in the energy required to complete the TDI distillation process.